This invention relates to signal processors for radar, sonar and like signal reception systems, and more particularly to sidelobe cancellation circuits for control of jamming, interference and other noise in such systems.
Still more particularly, the present invention constitutes a refinement of the basic intermediate frequency sidelobe canceller circuit disclosed and claimed in U.S. Pat. No. 3,202,990, which issued Aug. 24, 1965 to Paul W. Howells and is assigned to the assignee of the present application. The basic canceller loop of this Howells patent has proved to be effective against a variety of jamming and other interference sources, and it has been successfully and extensively used in many signal processor applications in both single loop and multiple loop versions as described in the patent. A modification of the basic multiple loop canceller is disclosed and claimed in the copending application of Kovarik, Howells and Applebaum, Ser. No. 165,259, filed Jan. 9, 1962, also of common assignment.
The provision of controllability or programmability of correlation signal weighting in accordance with the present invention affords significant enhancement of capabilities of sidelobe cancellers of the kind described in these earlier filed cases. Controllability of correlation signal weighting affords also greater adaptability and thus potentially greater applicability of such cancellers particularly in adverse operating environments such as those in which there are strong cluter returns or other returns from nearby highly reflective targets, or transmitted pulse interference due to cross-feed or leakage between the transmitter and receiver. In these situations the conventional sidelobe canceller will attempt to lock onto and cancel the clutter, large target return or transmitted pulse, and while cancellation of such interference is not objectionable in itself it serves no necessary purpose and the large transients introduced thereby into the cancellation loop may retard or introduce error into its response to the jamming signal input against which it is primarily intended to operate.
The results from the fact that in the interests of canceller selectivity and stability the time constant of the cancellation loop typically is relatively long as compared to the pulse repetition period (PRP), even spanning several PRP's in some cases. As a consequence, the canceller loop effectively reacts to an averaged value of the signal input over at least a substantial part of one pulse repetition period and sometimes over one or more complete PRP's. If over that period the input signal to the canceller loop includes, in addition to any jamming signal content, such additional or extraneous input as clutter, transmitted pulse energy or other non-jammer interference, the capability of the canceller loop to react to and cancel the jamming signal may be compromised thereby. The result of the correlation process within the canceller loop then will be to generate a correction or cancellation signal which is not precisely matched in amplitude and phase to the jammer signal alone but rather to time-averaged values of that signal plus the extraneous input, and such poorly matched correction signal will not completely cancel the interference.
While these problems can be alleviated to some extent by design for shorter loop time constants, this may as previously noted introduce other problems. Also, even if the loop time constant is made relatively very short, as for example some small fraction of the pulse repetition period, canceller operation still may be affected simply due to the presence of clutter or like signals in the loop which correlate and perturb the weights which would otherwise be optimized for cancellation of jamming.
Additionally, when operating against jamming signals the necessity for this finite loop time constant, and the finite time requirement for the proper cancellation signal to be generated to fully cancel the jamming signal, necessarily means that some small part of the jamming signal initially will get through the canceller before lock-on and complete cancellation is attained. This uncancelled signal is of such short duration and so attenuated by the canceller as usually not to be troublesome, but it may in some cases be necessary or desirable to cancel the jamming signal more completely, with little if any residue even during the lock-on period. This may be accomplished in accordance with the invention by substituting an externally supplied signal for the normal correlation signal in the canceller, with such externally supplied signal being pre-programmed to be of amplitude and phase such that the cancellation signal which will be generated within the loop in response thereto will be matched in time, amplitude and phase with the jamming signal. The jamming signal thus can be cancelled completely.